DOCS. 159, 160 DECEMBER 1915 157 Albert is now gradually entering the age at which I can mean very much to him. You can safely entrust him to me from time to time. Your relationship with him will not suffer from it in the least, as you will soon notice. For my influence is limited to the intellectual and the aesthetic. I want to teach him mainly to think, judge, and appreciate objectively. A number of weeks a year are necessary; I hope they will not only be enjoyable for him but also beneficial. A meeting of only a few days, however, is merely a thrill without any deeper value. Greetings to you and kisses for the boys from their Papa. 160. To Otto Naumann [Berlin,] 7 December 1915 Your Excellency, I have just been informed by my colleague Planck by telephone that he spoke with you about the Freundlich affair.[1] It affords me great pleasure to see that you have not lost sight of the matter, on account of which I had recently taken the liberty to call on you.[2] My colleague Planck recommended I now provide some more factual comments to orientate you on the state of the problems and on the related analyses to be taken up. The issue involves testing the so-called general theory of relativity. This the- ory bases itself on the assumption that no physical reality can be lent to time and space; it leads to a very specific theory of gravitation, according to which New- ton’s classical theory is valid only in admittedly superb first-order approximation. Verification of the results of this theory can be done only with methods used in as- tronomy. Three results could be found up to now for which a comparison against experiment is feasible at present. 1. The theory provides that the spectral lines in the light of fixed stars must be shifted slightly toward the red end of the spectrum against the corresponding spectral lines from terrestrial light sources, namely, the larger the mass of the light-emitting fixed star is, the more pronounced it is. This result was confirmed qualitatively by Mr. Freundlich using the already available observational data gathered mostly by American observatories.[3] Such a redshift was shown to be present on average, especially for fixed-star classes in which significant stellar masses had been determined through astronomical means. Establishing an average value through the measurement of numerous stars was necessary because the unknown individual motions of the separate stars also produce line shifts.